Electron spin resonance (ESR), also equivalently referred to herein as electron paramagnetic resonance (EPR), is a spectroscopic and imaging technique that is capable of providing quantitative information regarding the presence and concentration of a variety of magnetic species within a sample under test, e.g., a biological tissue sample. The valence electron(s) of a magnetic species possess unpaired spin angular momentum and thus, have net magnetic moments that tend to align along an externally applied magnetic field. This alignment process is known as magnetization. ESR is a measurement technique that relies on the external manipulation of the direction of this electron magnetization, also referred to as a net electronic magnetic moment. In a typical ESR experiment, a polarizing magnetic field B0 is applied to a sample to align the magnetic moments of the electrons along the direction of the magnetic field B0. Then, an oscillating magnetic field B1, often referred to as the transverse magnetic field, is applied along a direction that is perpendicular to the polarizing field B0. Usually the oscillating field B1 is generated using a microwave resonator (a coil or a transmission line) and is designed to excite the unpaired electrons by driving transitions between the different angular momentum states of the unpaired electron(s).
Currently there are two major techniques used to perform ESR spectroscopy. The first is a continuous wave (CW), frequency domain method and the second is a pulse-based, time domain technique. A CW spectrometer utilizes a continuous, narrow-band signal to create B1 and thus, energize unpaired electrons in the presence of the external DC magnetic field B0. In CW spectroscopy, an absorption spectrum of the sample is obtained by either sweeping the frequency of B1 while B0 is kept constant or by sweeping B0 while the frequency of B1 is kept constant. CW spectroscopy has been traditionally used for ESR because it is simpler in terms of circuitry and is able to detect samples even with very fast relaxation times (tens of nanoseconds). However, direct measurement of certain spin relaxation parameters, such as the longitudinal relaxation time, also referred to as the spin-lattice relaxation time (T1) and/or the transverse relaxation time, also referred to as the spin-spin relaxation time (T2) is feasible using time domain or pulse techniques. In pulse ESR, instead of sweeping a continuous signal, B1 is pulsed in a precisely designed pulse sequence to manipulate the direction of the spins of the unpaired electrons. The subsequent time-domain ESR signal emitted from the electrons as they relax back to their equilibrium state is then recorded by a receiver resonator. In pulsed ESR, wideband spectral information relating to the ESR samples may be reproduced using Fourier transform techniques applied to the ESR signal.
Presently, ESR imaging and spectroscopy are conducted using systems that employ a large number of discrete radiofrequency (RF) or microwave components. For example, current systems employ discrete RF sources, pulse generators, power amplifiers, lock-in amplifiers, resonators, mixers, analog-to digital converters, connecting cables, etc. However, as the instrument sizes exceed the characteristic wavelengths corresponding to the ESR experiment frequency (typically less than 1 meter, corresponding to a frequency of 300 MHz) the spectrometer/imager becomes sensitive to radiative effects and noise from the ambient RF radiation. This results in noisy and/or unstable data. Furthermore, the weight of the magnets and the RF components is typically hundreds of kilograms thereby prohibiting the portability of currently existing ESR spectrometers/imagers. Furthermore, the cost of building an ESR imager from discrete components can be prohibitively high. Finally large, discrete ESR imagers also have slow response times. This becomes a key limitation for time-domain imaging/spectroscopy, where the response time of the imager determines the shortest relaxation time that can be detected using time-domain ESR. Current in-vivo ESR imagers have response times that are limited to 1 microsecond or greater.